Earlier studies from this laboratory indicated that the hexose monophosphate shunt of human red cells is under severe intracellular restraint. The restraint is on glucose-6-phosphate dehydrogenase (G6PD), the first enzyme of the shunt, and cannot be attributed to limitations in substrate or to known inhibitors. Techniques of lysing and resealing red cells, coupled with use of mutant forms of G6PD, are allowing identification of the nature of the intracellular restraint. Related techniques are being used to investigate the nature of the susceptibility of G6PD-deficient people to hemolysis after exposure to fava beans and certain drugs.